The menstrual cycle functions to prepare the uterus to support a pregnancy. Instead of the body waiting until pregnancy occurs to start this process, each month the uterine lining gets ready to house a fertilized egg before an egg has even had the chance to become fertilized. That way, if an egg does get fertilized by a sperm, it has a welcoming environment waiting for it in the womb, even though the vast majority of the time such preparation will not be needed.  This is why pregnancy is usually calculated from the date of your last menstrual period, because that date marks the beginning of uterine preparation for pregnancy.

One menstrual cycle is counted from the first day of one menstrual period (day 1) to the first day of the next period. The average menstrual cycle is about 28 days long but normal cycles can range anywhere from 21 to 35 days (21 to 45 days in teen girls). Women may experience periods that are shorter or longer than this but this can be an indication of an underlying health problem. For the first few years after menstruation begins, longer cycles are common but the cycle tends to shorten and become more regular with age. Your cycle may not be the same length every month and the flow may also vary (within one period and between periods) from light to moderate to heavy. While most menstrual periods last from three to five days, anywhere from two to seven days is normal.

Ovulation, or the release of an immature egg from the ovaries, tends to occur in the middle of the menstrual cycle. In a 28 day cycle, ovulation occurs on day 14 following the start of menstruation. In a cycle that is shorter or longer than 28 days, ovulation will occur on a different day. In general, once ovulation has occurred, the cycle will last another 13-14 days and then if no pregnancy occurs, menstruation will begin. Thus, in a 45 day cycle, ovulation will occur around day 31.

The menstrual cycle occurs due to changes in the level of specific hormones (molecules produced in the body and used for signaling). In the first half of the menstrual cycle (from the first day of menstruation to the point of ovulation) the ovaries release increasing amounts of the hormone estrogen. This helps begin the process of thickening the uterine lining (called the endometrium) and prepares the egg for release. When the levels of estrogen reach their peak, ovulation occurs: an immature egg is released from the ovary. After being released, an immature egg travels in one of the two fallopian tubes (uterine tubes) which carries it to the uterus.

During the second half of the menstrual cycle, levels of another ovarian hormone called progesterone rise and exceed the levels of estrogen. Progesterone helps the uterine lining develop even further by thickening and developing glands that will support an embryo if the egg is fertilized. In order for conception to occur, the sperm and egg must meet in the middle third of the fallopian tube, a union that can only occur between 6 and 24 hours after ovulation. It can take sperm a few minutes or several hours to get up into the fallopian tube to the site of the egg and sperm can live inside the female reproductive tract for up to four days. So, a woman may become pregnant by having sex without contraception starting four days before ovulation and up to a day or so after ovulation. Remember though that it is very difficult to predict when ovulation will occur.